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Thread: voltage drop question

  1. #1
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    voltage drop question

    My customer wanted to delete a meter on a pole for a barn when he built his new house. It feeds a 100 amp panel in a barn at least 350 feet away from the meter on a pole. ( I'm going to try and measure that distance today) They went underground with about 3/0 Al, looks like. I measured 123 volts in the barn the other day. Now that his house is built, I ran from a 200 amp house sub panel (measuring 122.3 volts) with a 20 foot #1 Al to outside wall, then to 1/0 Al underground for 190 feet to the pole. I'm getting close to deleting the meter, but my voltage at the pole from the new house is measuring 118 volts and 240 volts. (3.5%) I could still change my #1 to 1/0 without too much trouble if that would help. Or I could put a 90 amp breaker instead of a 100. Im thinking on temporarily disconnecting the meter source and connecting my new house source and checking the voltage at the barn. Any suggestions ? Thank you.

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    You need to know the load at your barn. Simply changing the 100 to a 90 does nothing for you if the VD is excessive with a 50 amp load.

    Do you have sensitive equipment that requires no less than 115.23 volts or can you live with momentary dimming of lights when the beer cooler starts?

    Changing to 1/0 will help.
    Tom
    TBLO

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    190712-0730 EDT

    Stevenfyeager:

    ptonsparky gave you a good answer, you need to know current changes.

    I can not understand your question or layout. Also you need to paragraph your writing.

    It appears you have a barn, a house, a pole, and somewhere there is a transformer.

    I have to assume it is a pole transformer and mounted on said pole where meter is. The meter is basically not important.

    At a center tapped transformer I might expect the two nominal 120 V readings to be within possibly about 0.2 V when unloaded. You read a 2 V difference.

    What does the barn have to do with the house? Does the house have a main panel? Does the barn have a main panel? What is the wiring from the pole transformer to the house, and to the barn? How far is the barn from the house?

    For a 100 A load change on some circuit on a main panel how much voltage change do you want to tolerate at that main panel? A 1 V change at 120 V to an incandescent bulb does not produce a significant flicker, 5 V does. Do some experiments with different voltage changes to an incandescent to try to determine your criteria.

    Light flicker from a motor starting may be something you have to tolerate.

    .

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    Quote Originally Posted by ptonsparky View Post
    You need to know the load at your barn. Simply changing the 100 to a 90 does nothing for you if the VD is excessive with a 50 amp load.

    Do you have sensitive equipment that requires no less than 115.23 volts or can you live with momentary dimming of lights when the beer cooler starts?

    Changing to 1/0 will help.
    Thank you, the 100 amp barn panel supplies an RV right now is about all. It is not a cattle farm or real farm. They have a few horses.
    I didnt think changing 100 amp breaker to a 90 would help, but in the voltage drop equation, or calculator, it does help, correct ?
    I measured the distance between the metered pole and barn yesterday. 440 feet. (I'd say 3/0 or 4/0 Al) I measure no voltage drop, from the pole to barn 123 volts, both places.
    I'm curious what it will measure when I delete the meter and power it from the house 190 feet from the pole with my 1/0 Al. I will temporarily connect that and check that when I get a chance. Thank you.

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    Quote Originally Posted by Stevenfyeager View Post
    I didnt think changing 100 amp breaker to a 90 would help, but in the voltage drop equation, or calculator, it does help, correct ?....
    You calculate voltage drop based on the actual number of amps being drawn out there at the barn, not the size of the breaker you use to send power to the barn.
    If you don't think too good, don't think too much.

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    Voltage drop only comes into play when current is flowing (there is an active load) and the amount of voltage drop is proportional to the amount of current being drawn. If there is no load (no current draw) then there is no voltage drop. You could have 1,000 feet of 18AWG wire connected to a 120V panel and measure 120V at the other end if there's no load (assuming a high-impedance volt meter is used to take the measurement).

    From your narrative, it sounds like your new wire (from the house to the pole) is not yet connected to anything. So it makes no sense when you say your new wire is getting 122.3V at the house panel, but you're only measuring 118V at the end of your wire at the pole. If there is no load, you should be reading 122.3V at the end of that wire.

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